DAEHAN HWAHAK HWOEJEE
(Journal of theKorean Chemical Society) Vol. 19, No. 6, 1975
Printed in Republic of Korea
화학흡착에 의한 백금 및 백금-텅스텐 진공 증착막의 표면적 측정 *
*This work was doneat Mobil Research & Develo pment corporation, Princeton, N. J., U. S. A.
全 學 濟
한국과학원 화학 및 화학공학과 (1975. 3. 31 접수)
Surface Area Measurement by Chemisorption of Gases on Vaccum Evaporated thin Film
of Platinum-Tungsten Film-
Hakze Chon
Korea Advanced Institute of Science, Seoul, Korea
(Received March31, 1975)
요 약. 백금 백금"■텅스텐 증착막에 의한 수소, 일산화탄소의 화학홉착과 화학흡착시킨 산소의 수소 titration을 110°C 에서 실험해본 결고｝, H2/O2 titration 방법에 의한 백금-텅스텐 증착막에서 의 백금 표면적 측정이 가능한 것 같다.
ABSTRACT. The chemisorption of hydrogen and carbon monoxide, as well as the hydrogen titration of prechemisorbed oxygen was studied at 110 °C on evaporated platinum and platinum
tungsten films. The results suggest that hydrogen titration of prechemisorbed oxygen may be used to determine the platinum surface area of platinum-tungsten film.
INTRODUCTION
Selective chemisorption of gases, mainly hydr
ogen and carbon monoxide, has ueen used to determine the surface area of metal dispersed on different supports. Selective chemisorption seems to be the only technique applicable to measure the metal dispersion in case the metal crystallitesize is too small to be measured by X-ray technique.
Chon, et al.1 studied the calorimetrically indicaed titration of oxygen and hydrogen on platinum black at 0°C and interpreted the results on the hydrogen titration of oxygen-prechemisorbed
platinum in terms of the following reaction.
Pt-0(5)+*H2 (g)—>Pt—H(s) +H20(g) Benson & Boudart2 applied the hydrogen-oxygen titration method to the measurement of supported platinum surface area. H2/O2 titration on Pt and on supported Rh is reported elsewhere3~5. The advantage of the titration technique lies in the increase of sensitivity and in the simplicity of the measurement. In this study the possibility of determining the platinum surface area of the platinum-tungsten film by using the titration of chemisorbed oxygen with hydrogen is investi
gated.
—420 —
全學濟
EXPERIMENTAL
Thin films of platinum and platinum-tungsten are being vacuum deposited on the surface of glass vessels. The source of vapor is a U shaped platinum wire (B & S 26 gauge) or a platinum
tungsten wire(0. 016/z) spot welded to tungsten lead wires. Fig. 1 shows the arrangement for film deposition. The films are pretreated at 450
°C in high vacuum before adsorption measure
ments. Hydrogen(99. 999 %), carbon monoxide (99. 9 %) and krypton(99. 995 %), research grade from Matheson Gas Products, and oxygen (99.
996 %), research grade from Lif-O-Gen. Inc.
were passel through a liquid nitrogen trap before use. Chemisorption of hydrogen and carbon monoxide and H2/O2 titration was studied at 110
°C. The adsorption vessel which is provided with a cold finger immersed in liquid nitrogen to trap water or carbon dioxide formed. Pressure is measured with a capacitance manometer or a thermistor vacuum gauge. The thermistor vacu
um gauge was constructed using a matched pair of thermistors and was calibrated against a McLeod gauge for individual gases.
RESULTS and niSCUSSION
by the oxygen-prechemisorbed surface is due to the reaction of chemisrobed oxygen with hy
drogen and subsequent chemisorption of hydrogen on the oxygen-free platinum surface (0. 20 x 10-1 cc). The difference in the amount of hydrogen uptake between the oxygen-prechemisorbed sur
face and the oxygen free surface (1. 82 x 10-3 cc) corresponds to 42cm2 of platinum surface area using 8. 4*** as an average area for a surface platinum atom, assuming the surface reaction of PtO (s') +H2—>Pt-T-H2O. Studies on CO chemisorp
tion on platinum/ silica shows that the ratio (Pt atom exposed) / (CO molecules adsorbed) varies from〜1 to〜2 with increasing platinum content6. It is suggested that the Pt/CO ratio depends on the crystallite size, and that CO may be present in bridged form on large crystallites and linear form on highly dispersed platinum.
On evaporated platinum films it is assumed that
*This area was obtained by averaging the area of the platinum atoms present in the (100) (110) and (ill) crystallographic planes.
iO 20 30 40
Pressure(/J) Fig. 2. Uptake of hydrogen and carbon monoxide by platinium film and ofhydrogen byoxigen-prechem
isorbed platinum surface at 110 °C.
Fig. 2 shows the uptake of hydrogen and carbon monoxide by platinum film and of hydrogen by oxygen-prechemisorbed platinum surface. The uptake of hydrogen(2. 02 x 10-3 cc)
Vol. 19, No. 6, 1975
422 화학홉-착에 의한 백금 및 배금-텅스텐 진공증착막의 표먼적 측정
! Platinum jPlatinuni-Tungsten Table 1. Surface area of the platinium and platinum
tungsten Films.
Hydrogen titration 42cm2 1 25 cm2 CO Chemisorpton 49 cm2 丨 28cm2 BET(Krypton) 52cm2 65 cm2
CO is assumed that CO is chemisorbed primarily in bridged form. Assuming, the bridged form, uptake of 1.05乂10一3(口하 of CO by platinum film corresponds to 49 cm2 of platinum surface area. The slight discrepancy in the value of platinum surface area obtained from H2/O2 titration and CO chemisorption may be attributed to the presence of two different forms of CO on the surface.
The uptake of hydrogen by oxygen prechemi
sorbed platinum-tungsten films at 110° C is much less(l. 2x 10~3 cm3) 아the uptake by platinum films (2. 02 x 10"3), even-though the krypton BET area of platinum-tungsten film is larger (65 cm2) than that of platinum (52 cm2) as shown in the Table 1. The Hydrogen titration of pre
chemisorbed oxygen takes place readily even at room temperature. If we assume that the oxygen chemisorbed on tungsten is not removed by hy
drogen at 110 °C, the hydrogen uptake of 1.2 x IO-3 cm~3 by oxygen prechemisorbed platimum- tungsten may be regarded as the result of hydrogen titration of oxygen-prechemisorbed platinum
sites. The uptake of L 2 x 10~3 cm~3 then corre
sponds to 24cm2 of platinum area.
On a platinum-tungsten film at 110 °C the rapid adsorption of CO is followed by a slow uptake. The slow uptake of CO was not obse rved for platinum film at 110°C. The nature of the slow uptake is not clear. If we take the amount of rapid uptake of CO as the amount taken up by platinum, uptake of 0 58 x 10"3cc corresponds to 25 cm2 of platinum area. This compares well with the value obtained from H2 /O2 titration on platinum-tungsten film.
The results suggest that the selective hydro용en titration of chemisorbed oxygen may be used to determine the platinum surface area o£ platinum- tungsten films.
REFERENCES
1. H. Chon, R. A. Fisher and J. G. Aston, J. Amer. Chem. Soc., 82, 1055(1960).
2. John E. BensonandMichel Boudart, J. Catalysis, 4, 704〜710(1965)
3. M. A. Vannice, J. E. Benson, and M. Boudart, J. Catalysis, 16, 348〜356(1970).
4. M. A나itar(Miss) and F.C. Tompkins, Trans.
Faraday Soc.3 67, 2454(1971)
5- Sieghard E. Wanke and Neil A. Daugharty J. Catalysis, 24, 367—384(1972)
6. T. A. Dorling and R. L. Moss, J. Catalysis^
7, 378^385(1967).
Journal of the KoreanChemicalSociety
